Secondary electron discharge device



v P. W. STUTSMAN SECONDARY ELECTRON DISCHARGEDEVICE Aug. 7, 1945.

Filed Jan. 5, 1942 M W m F Tie].

. I I 9 L 1,

I II \NVAE'NTOR PAUZ W. S'ruTsMAN, TTY.

" Patented 17 SECONDARY ELECTRON DISCHARGE DEVICE Paul Stutsman, Waltham, assignor to Raythcon Manufacturing Company, Newton, Masa, a corporation of Delaware Application January 3, 1942, Serial No. 425.545

2 Claims.

operated cold and the electron emission therefrom produced by electron bombardment with resultant secondary electron emission. Heretofore such cathodes have been difficult to activate so as to produce the proper amounts of secondary electron emission, the characteristics of such cathodes have varied during the life thereof. and the useful life of such cathodes has been undesirably short.

An object of this invention is to increase the sensitivity, constancy. and life of secondary electron emission devices of the general type as described above.

Another object is to devise a secondary electron emissi on' cathode which is operated at an elevated temperature under conditions resulting in relatively poor but nevertheless appreciable thermionic emission and excellent secondary electron emission.

The foregoing and other objects of this invention will be best understood from the following description of an exemplification thereof, reference being had to the accompanying drawing, wherein:

Fig. 1 is a cross-section of an electron discharge tube embodying my invention shown in connection with a diagrammatic circuit with which said tube may be utilized; and

Fig. 2 is a cross-section taken along line 2-2 of Fig. 1.

The electronic discharge tube illustrated comprise an evacuated envelope I containing two of my novel cathodes 2 and 3, spaced apart therein on opposite sides of an anode ring 4. Said anode is disposed so that electrons emitted from either or both of the cathodes may oscillate freely through the opening therein.

Each of my novel cathodes 2 and 3 is made preferably in the form of a cup of a metal, such as tantalum, tungsten or molybdenum. The material which I prefer to use in this relationship is tantalum because of its excellent properties when utilized in accordance with my invention. The cup 5 may be provided with a back cover 6 defining a chamber within which is disposed a heating filament l. The front face of the cup 5 is provided with a coating of an oxide or oxides of one or more of the alkaline earth metals.

Preferably this coating is initially applied in the form of a mixture of barium and strontium carbonates which during the fabrication of the device is broken down into the oxides. Each cathode is supported by a pair of support leads 8 and I0 sealed through stems l0 disposed at the opposite ends of the envelope I. One endof the heater filament l is electrically connected to and supported by the cup 5. The other end of the filament I is connected to a lead wire II which extends through an opening centrally disposed in the cover 5, and likewise sealed through the respective stem Ill. The anode 4 may be conveniently supported by a lead-in conductor 4' sealed through the side wall of the envelope I. A solenoid 12 may be disposed around the envelope i so that when energized it creates a magnetic field extending longitudinally between the two cathodes 2 and 3.

Each of the filaments I may be heated by heating current conveniently supplied from heating transformers I! provided with primary windings l4 adapted to be connected to a suitable source of alternating current, and with secondary windings l5 each connected to the lead wires Ill and II of the respective cathode.

The cathodes 2 and 3 are connected through conductors l6 and I! to the opposite ends of an induction coil l8. Across the coil I8 is connected a condenser l9 which may be adjustable to permit tuning of the parallel inductance and capacity circuit. The midpoint of the coil Ii is connected through a suitable source of direct current 20 and one or more choke coils 2| to the lead-in conductor 4', and thus to the anode 4. The choke coil 2| presents a high impedance to the high frequency oscillations which are adapted to be set up in the parallel inductance capacity circuit i8-l9. Polarity of the source 20 is chosen so as to make the anode 4 positive with respect to the cathodes 2 and 3. Coupled to the coil I8 is an output coil 22, whereby the oscillations generated may be led to some suitable insulation circuit. The coil I! may be energized from a suitable source of current 23 connected across the ends of said coil in series with the current-limiting resistance 24.

When the device as described above is energized and the circuit l8l9 is tuned to a frequency whose half period is approximately equal to the flight time of electrons travelling from one cathode to the other, strong oscillations will be generated by the arrangement, said oscillations having the frequency as determined by the tuned circuit lB-IQ. Under these conditions a cloud of electrons will oscillate back and forth through the opening in the anode ring I, said cloud of electrons colliding at each end of its travel with one of the cathodes I or 3 with sufficient energy to liberate large numbers of secondary electrons from the activated cathode surfaces.

The temperature of operation of the heater I is so selected that the coated surfaces of the cathode surface; I and 3 are raised to a temperature at which appreciable thermionic emission is produced. However, under the conditions of operation, this temperature is such as to make the surfaces relatively poor thermionic emitters so that the total thermionic emission produced from these surfaces is substantially less than the peak currents which the device is called upon to supply. I have found, however, that at these elevated temperatures the cathode surfaces, as described above, are excellent secondary electron emitters. I believe that one reason for such enhanced secondary electron emission is that at these temperatures the tantalum of the cathode operates to reduce some of the alkaline earth oxides, liberating minute and presumably monatomic films of barium or strontium or both on the cathode surfaces. These alkaline earth metals in contact with the tantalum evidently operate as extraordinarily effective secondary electron emitters. The characteristics of cathode surfaces of this type remain constant within a remarkably narrow range throughout the life of the device, and also the cathodes have exceptionally long lives as compared with those heretofore secured in arrangements of this kind.

course it is to be understood that this invention is not limited to the particular details as described above, as mamr equivalents will suggest themselves to those skilled in the art. For example, other types of cathode surfaces might be devised. Free barium may be vaporized from an adjacent source onto a bare cathode surface. Also alloys of various kinds which are good secondary electron emitters may constitute the surface of the cathode. I believe that in each instance elevated temperatures of operaticm in accordance with my invention will produce the various beneficial results which I have described above.

What is claimed is:

1. An electron discharge device system comprising a pair of cathodes and an anode interposed between said cathodes and provided with an opening through which electrons may pass from one cathode to the other, said cathodes during normal operation being at elevated temperatures and provided with active surfaces which at said temperatures are appreciable but relatively poor thermionic emitters with thermionic emisaion which is substantially smaller than that required to carry the normal load current of said device and also good secondary electron emitters, and an oscillatory circuit connected to said cathodes and anode for producing a cloud of electrons and for oscillating said cloud of electrons between said cathodes through said opening with suiiicient energy to cause electrons thereof to collide with said cathodes to liberate suilicient secondary electrons therefrom to produce a total electron emission capable of carrying said normal load current, the space between said cloud of electrons and said cathodes being unimpeded to cause said collisions to occur fully.

2. An electron discharge device comprising a pair of cathodes and an anode interposed between said cathodes and provided with an opening through which electrons may pass from one cathode to the other, said cathodes during normal operation being at elevated temperatures and provided with active surfaces which at said temperatures are appreciable but relatively poor thermionic emitters with thermionic emission which is substantially smaller than that required to carry the normal load current of said device and also good secondary electron emitters, the space between said cathodes being unimpeded.

PAUL W. STUTSMAN. 

